Flowers originally evolved for cross-pollination, but there is always the poten-
tial for self-pollination in a hermaphrodite flower and some species can produce
full seed set from self-fertilization (Topic H2). They do not require any of the
adaptations for pollen dissemination or receipt and many have small flowers
with a limited amount of pollen. A much larger number of plants are self-
compatible and can self-fertilize, although they normally cross-fertilize. In these,
self-fertilization frequently occurs only if cross-fertilization has failed, often with
the pollen tubes of a plant’s own grains growing slower than those from another
plant, or the pollen only reaching receptive stigmas as the flower ages.
The advantages of self-fertilization are that no external pollinating agent is
needed so flowers can be small, providing no attractive parts and producing
only enough pollen to fertilize their own ovules. Seed set is assured, and with
fewer resources being used in flowers, it may be able to flower at an earlier
developmental stage (Topic H2). This has particular advantages in colonizing
situations and in ephemeral habitats.
Constant selfing will mean that the plant loses much of its genetic variation
and, after a few generations, will be homozygousat almost all its genes, i.e. each
gene copy will be identical. This can lead to deleterious genes being manifest
and much mortality in the early generations, but once any deleterious genes
have disappeared the plants will be all identical. This means that only proven
successful plants will be produced and no seeds are ‘wasted’ on poor gene
combinations. Many habitual selfers are polyploid so have several copies of
each gene anyway, avoiding some of the problems of homozygosity.
The disadvantages of self-fertilization are the lack of new gene combinations
leading to much reduced variation, dependent only on new mutations. This
means that they may not be able to adapt to a changing environment, particu-
larly important in developing resistance to herbivores or pathogens or in colo-
nizing new sites. It also means that they never have the advantages of hybrid
vigor which probably arise from a plant having two different forms of certain
genes in each cell (Topic M5).
In plants that may cross or self-fertilize there is nearly always a disadvantage
to selfing. This may be manifest in lower seed set, smaller or less resistant seeds,
difficulties in germination, ability of the seedling to establish, vigor and survival
of the offspring or their ability to reproduce or any combination of these
features. This is known as inbreeding depression. Even in most habitual self-
fertilizing plants studied, experimental cross-pollination has led to more
vigorous plants than selfing and almost all plants do occasionally cross-fertilize.Self and cross
fertilization
186 Section L – Reproductive ecology